mvgam case study 2: multivariate models
Nicholas Clark (n.clark@uq.edu.au)
In this example we will examine multivariate forecasting models using mvgam, which fits GAMs using MCMC sampling via the JAGS software (Note that JAGS is required; installation links are found here). First a simulation experiment to determine whether mvgam's inclusion of complexity penalisation works by reducing the number of un-needed dynamic factors. In any factor model, choosing the appropriate number of factors K can be difficult. The approach used by mvgam is to estimate a penalty for each factor that squeezes the factor's variance toward zero, effectively forcing the factor to evolve as a flat white noise process. By allowing each factor's penalty to be estimated in an exponentially increasing manner (following Welty, Leah J., et al. Bayesian distributed lag models: estimating effects of particulate matter air pollution on daily mortality Biometrics 65.1 (2009): 282-291), we hope that we can guard against specying too large a K. But we should test this property to see how it behaves in different scenarios. Begin by simulating 6 series that evolve with a shared seasonal pattern and that depend on 2 latent random walk factors. Each series is 80 time steps long, with a seasonal frequency of 12. We give the trend moderate importance by setting trend_rel = 0.6 and we allow each series' observation process to be drawn from slightly different Poisson distributions
set.seed(500)
library(mvgam)## Loading required package: mgcv## Loading required package: nlme## This is mgcv 1.8-33. For overview type 'help("mgcv-package")'.## Loading required package: rjags## Loading required package: coda## Linked to JAGS 4.3.0## Loaded modules: basemod,bugs## Loading required package: parallel## Loading required package: runjags## Warning: package 'runjags' was built under R version 4.0.5dat <- sim_mvgam(T = 80, n_series = 6, n_trends = 2,
family = "poisson", mu_obs = runif(8,
4, 6), trend_rel = 0.6, train_prop = 0.8)## Warning: replacing previous import 'lifecycle::last_warnings' by
## 'rlang::last_warnings' when loading 'tibble'## Warning: replacing previous import 'lifecycle::last_warnings' by
## 'rlang::last_warnings' when loading 'pillar'## Registered S3 method overwritten by 'quantmod':
## method from
## as.zoo.data.frame zooHave a look at the series
par(mfrow = c(3, 2))
for (i in 1:6) {
plot(dat$data_train$y[which(as.numeric(dat$data_train$series) ==
i)], type = "l", ylab = paste("Series",
i), xlab = "Time")
}
par(mfrow = c(1, 1))Clearly there are some correlations in the trends for these series. But how does a dynamic factor process allow us to potentially capture these dependencies? The below example demonstrates how. Essentially, a dynamic factor is an unobserved (latent) random process that induces correlations between time series via a set of factor loadings (\(\beta\)) while exercising dimension reduction. The loadings represent constant associations between the observed time series and the dynamic factor, but each series can still deviate from the factor through its error process and its associations with other factors (if we estimate >1 latent factor in our model).
A challenge with any factor model is the need to determine the number of factors K. Setting K too small prevents temporal dependencies from being adequately modelled, leading to poor convergence and difficulty estimating smooth parameters. By contrast, setting K too large leads to unnecessary computation. mvgam approaches this problem by formulating a prior distribution that enforces exponentially increasing penalties on the factor variances to allow any un-needed factors to evolve as flat lines. Now let's fit a well-specified model for our simulated series in which we estimate random intercepts, a shared seasonal cyclic smooth and 2 latent dynamic factors
mod1 <- mvjagam(data_train = dat$data_train,
data_test = dat$data_test, formula = y ~
s(series, bs = "re") + s(season,
bs = c("cc"), k = 12), knots = list(season = c(0.5,
12.5)), use_lv = T, n_lv = 2, family = "poisson",
trend_model = "RW", chains = 4, burnin = 4000)## NOTE: Stopping adaptationLook at a few plots. The estimated smooth function
plot_mvgam_smooth(object = mod1, series = 1,
smooth = "season")
And the true seasonal function in the simulation
plot(dat$global_seasonality[1:12], type = "l")
Check whether each factor was retained using the plot_mvgam_factors function. Here, each factor is tested against a null hypothesis of white noise by calculating the sum of the factor's 1st derivatives. A factor that has a larger contribution to the series' latent trends will have a larger sum, both because that factor's absolute magnitudes will be larger (due to the weaker penalty on the factor's precision) and because the factor will move around more. By normalising these estimated first derivative sums, it should be apparent whether some factors have been dropped from the model. Here we see that each factor is contributing to the series' latent trends, and the plots show that neither has been forced to evolve as white noise
plot_mvgam_factors(mod1)
Now we fit the same model but assume that we no nothing about how many factors to use, so we specify the maximum allowed (the total number of series; 8). Note that this model is computationally more expensive so it will take longer to fit
mod2 <- mvjagam(data_train = dat$data_train,
data_test = dat$data_test, formula = y ~
s(series, bs = "re") + s(season,
bs = c("cc"), k = 12), knots = list(season = c(0.5,
12.5)), use_lv = T, n_lv = 6, family = "poisson",
trend_model = "RW", chains = 4, burnin = 4000)## NOTE: Stopping adaptationThe same plots as model 1 to see if this model has also fit the data well
plot_mvgam_smooth(object = mod2, series = 1,
smooth = "season")
plot(dat$global_seasonality[1:12], type = "l")
Examining the factor contributions gives us some insight into whether we set n_lv larger than we perhaps needed to. These contributions can be interpreted similarly to ordination axes when deciding how many latent variables to specify
plot_mvgam_factors(mod2)
The very weak contributions by some of the factors are a result of the penalisation, which will become more important as the dimensionality of the data grows. Now onto an empirical example. Here we will access monthly search volume data from Google Trends, focusing on relative importances of search terms related to tick paralysis in Queensland, Australia
library(tidyr)
if (!require(gtrendsR)) {
install.packages("gtrendsR")
}
terms = c("tick bite", "tick paralysis",
"dog tick", "paralysis tick dog")
trends <- gtrendsR::gtrends(terms, geo = "AU-QLD",
time = "all", onlyInterest = T)Google Trends modified their algorithm for extracting search volume data in 2012, so we filter the series to only include observations after this point in time
gtest <- trends$interest_over_time %>% tidyr::spread(keyword,
hits) %>% dplyr::select(-geo, -time,
-gprop, -category) %>% dplyr::mutate(date = lubridate::ymd(date)) %>%
dplyr::mutate(year = lubridate::year(date)) %>%
dplyr::filter(year > 2012) %>% dplyr::select(-year)Convert to an xts object and then to the required mvgam format, holding out the final 10% of observations as the test data
series <- xts::xts(x = gtest[, -1], order.by = gtest$date)
trends_data <- series_to_mvgam(series, freq = 12,
train_prop = 0.9)Plot the series to see how similar their seasonal shapes are over time
plot(series, legend.loc = "topleft")
Now we will fit an mvgam model with shared seasonality and random intercepts per series. Our first attempt will ignore any temporal component in the residuals so that we can identidy which GAM predictor combination gives us the best fit, prior to investigating how to deal with any remaining autocorrelation. We assume a Negative Binomial distrubution with series-specific overdispersion parameters for the response. We use a complexity-penalising prior for the overdispersion, which allows the model to reduce toward a more simple Poisson observation process unless the data provide adequate information to support overdispersion. Also note that any smooths using the random effects basis (s(series, bs = "re") below) are automatically re-parameterised to use the non-centred parameterisation that is necessary to help avoid common posterior degeneracies in hierarchical models. This parameterisation tends to work better for most ecological problems where the data for each group / context are not highly informative, but it is still probably worth investigating whether a centred or even a mix of centred / non-centred will give better computational performance. We suppress the global intercept as it is not needed and will lead to identifiability issues when estimating the series-specific random intercepts
trends_mod1 <- mvjagam(data_train = trends_data$data_train,
data_test = trends_data$data_test, formula = y ~
s(series, bs = "re") + s(season,
k = 12, m = 2, bs = "cc") - 1,
knots = list(season = c(0.5, 12.5)),
trend_model = "None", family = "nb",
chains = 4, burnin = 8000)## NOTE: Stopping adaptationGiven that these series could potentially be following a hierarchical seasonality, we will also trial a slghtly more complex model with an extra smoothing term per series that allows its seasonal curve to deviate from the global seasonal smooth. Ignore the warning about repeated smooths, as this is not an issue for estimation.
trends_mod2 <- mvjagam(data_train = trends_data$data_train,
data_test = trends_data$data_test, formula = y ~
s(season, k = 12, m = 2, bs = "cc") +
s(season, series, k = 5, bs = "fs",
m = 1), knots = list(season = c(0.5,
12.5)), trend_model = "None", family = "nb",
chains = 4, burnin = 8000)## NOTE: Stopping adaptationHow can we compare these models to ensure we choose one that performs well and provides useful inferences? Beyond posterior retrodictive and predictive checks, we can take advantage of the fact that mvgam fits an mgcv model to provide all the necessary penalty matrices, as well as to identify good initial values for smoothing parameters. Because we did not modify this model by adding a trend component (the only modification is that we estimated series-specific overdispersion parameters), we can still employ the usual mgcv model comparison routines
anova(trends_mod1$mgcv_model, trends_mod2$mgcv_model,
test = "LRT")AIC(trends_mod1$mgcv_model, trends_mod2$mgcv_model)summary(trends_mod1$mgcv_model)##
## Family: Negative Binomial(13.752)
## Link function: log
##
## Formula:
## y ~ s(series, bs = "re") + s(season, k = 12, m = 2, bs = "cc") -
## 1
##
## Approximate significance of smooth terms:
## edf Ref.df Chi.sq p-value
## s(series) 3.999 4 26416 <2e-16 ***
## s(season) 6.662 10 67133 <2e-16 ***
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## R-sq.(adj) = 0.787 Deviance explained = 80.8%
## -REML = 1310.3 Scale est. = 1 n = 404summary(trends_mod2$mgcv_model)##
## Family: Negative Binomial(19.914)
## Link function: log
##
## Formula:
## y ~ s(season, k = 12, m = 2, bs = "cc") + s(season, series, k = 5,
## bs = "fs", m = 1)
##
## Parametric coefficients:
## Estimate Std. Error z value Pr(>|z|)
## (Intercept) 2.6783 0.4821 5.555 2.78e-08 ***
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## Approximate significance of smooth terms:
## edf Ref.df Chi.sq p-value
## s(season) 6.429 10 105.4 <2e-16 ***
## s(season,series) 13.020 19 1311.2 <2e-16 ***
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## R-sq.(adj) = 0.83 Deviance explained = 84.1%
## -REML = 1284.1 Scale est. = 1 n = 404WE can also use JAGS routines for interrogating models. Which model minimises in-sample DIC?
dic(trends_mod1)## Mean deviance: 2567
## penalty 17.32
## Penalized deviance: 2584dic(trends_mod2)## Mean deviance: 2496
## penalty 28.27
## Penalized deviance: 2524Model 2 seems to fit better so far, suggesting that hierarchical seasonality gives better performance for these series. But a problem with both of the above models is that their forecast uncertainties will not increase into the future, which is not how time series forecasts should behave. Here we fit Model 2 again but specifying a time series model for the latent trends. We assume the dynamic trends can be represented using latent factors that each follow an AR1 process, and we will rely on the exponential penalties to help regularise any un-needed factors by setting n_lv = 4
trends_mod3 <- mvjagam(data_train = trends_data$data_train,
data_test = trends_data$data_test, formula = y ~
s(season, k = 12, m = 2, bs = "cc") +
s(season, series, k = 5, bs = "fs",
m = 1), knots = list(season = c(0.5,
12.5)), trend_model = "AR1", use_lv = T,
n_lv = 4, family = "nb", chains = 4,
burnin = 8000)## NOTE: Stopping adaptationHave a look at the returned JAGS model file to see how the dynamic factors are incorporated. Notice that the precisions of factors, together with each factor's set of loadings, are penalised using a regularised horseshoe prior
trends_mod3$model_file## [1] "JAGS code generated by package mvgam"
## [2] ""
## [3] "GAM formula:"
## [4] "y ~ s(season, k = 12, m = 2, bs = cc) + s(season, series, k = 5, bs = fs, m = 1)"
## [5] ""
## [6] "Trend model:"
## [7] "AR1"
## [8] ""
## [9] "Required data:"
## [10] "integer n; number of timepoints per series"
## [11] "integer n_series; number of series"
## [12] "matrix y; time-ordered observations of dimension n x n_series (missing values allowed)"
## [13] "matrix ytimes; time-ordered n x n_series matrix (which row in X belongs to each [time, series] observation?)"
## [14] "matrix X; mgcv GAM design matrix of dimension (n x n_series) x basis dimension"
## [15] "matrix S1; mgcv smooth penalty matrix S1"
## [16] "vector zero; prior basis coefficient locations vector of length ncol(X)"
## [17] "vector p_coefs; vector (length = 1) of prior Gaussian means for parametric effects"
## [18] "vector p_taus; vector (length = 1) of prior Gaussian precisions for parametric effects"
## [19] "vector sp; inverse exponential location priors for smoothing parameters: s(season); s(season,series)1; s(season,series)2"
## [20] "___________values ranging 5 - 50 are a good start"
## [21] ""
## [22] ""
## [23] "#### Begin model ####"
## [24] "model {"
## [25] ""
## [26] "## GAM linear predictor"
## [27] "eta <- X %*% b"
## [28] ""
## [29] "## mean expectations"
## [30] "for (i in 1:n) {"
## [31] "for (s in 1:n_series) {"
## [32] "mu[i, s] <- exp(eta[ytimes[i, s]] + trend[i, s])"
## [33] "}"
## [34] "}"
## [35] ""
## [36] "## latent factors evolve as time series with penalised precisions;"
## [37] "## the penalty terms force any un-needed factors to evolve as flat lines"
## [38] "for (j in 1:n_lv) {"
## [39] "LV[1, j] ~ dnorm(0, penalty[j])"
## [40] "}"
## [41] ""
## [42] "for (i in 2:n) {"
## [43] "for (j in 1:n_lv) {"
## [44] "LV[i, j] ~ dnorm(ar1[j]*LV[i - 1, j], penalty[j])"
## [45] "}"
## [46] "}"
## [47] ""
## [48] "## AR components"
## [49] "for (s in 1:n_lv) {"
## [50] "ar1[s] ~ dnorm(0, 10)"
## [51] "}"
## [52] ""
## [53] "## shrinkage penalties for each factor squeeze the factor to a flat line and squeeze"
## [54] "## the entire factor toward a flat white noise process if supported by"
## [55] "## the data. The prior for individual factor penalties allows each factor to possibly"
## [56] "## have a relatively large penalty, which shrinks the prior for that factor's variance"
## [57] "## substantially. Penalties increase exponentially with the number of factors following"
## [58] "## Welty, Leah J., et al. Bayesian distributed lag models: estimating effects of particulate"
## [59] "## matter air pollution on daily mortality Biometrics 65.1 (2009): 282-291."
## [60] "pi ~ dunif(0, n_lv)"
## [61] "X2 ~ dnorm(0, 1)T(0, )"
## [62] ""
## [63] "# eta1 controls the baseline penalty"
## [64] "eta1 ~ dunif(-1, 1)"
## [65] ""
## [66] "# eta2 controls how quickly the penalties exponentially increase"
## [67] "eta2 ~ dunif(-1, 1)"
## [68] ""
## [69] "for (t in 1:n_lv) {"
## [70] "X1[t] ~ dnorm(0, 1)T(0, )"
## [71] "l.dist[t] <- max(t, pi[])"
## [72] "l.weight[t] <- exp(eta2[] * l.dist[t])"
## [73] "l.var[t] <- exp(eta1[] * l.dist[t] / 2) * 1"
## [74] "theta.prime[t] <- l.weight[t] * X1[t] + (1 - l.weight[t]) * X2[]"
## [75] "penalty[t] <- max(0.0001, theta.prime[t] * l.var[t])"
## [76] "}"
## [77] ""
## [78] "## latent factor loadings: standard normal with identifiability constraints"
## [79] "## upper triangle of loading matrix set to zero"
## [80] "for (j in 1:(n_lv - 1)) {"
## [81] "for (j2 in (j + 1):n_lv) {"
## [82] "lv_coefs[j, j2] <- 0"
## [83] "}"
## [84] "}"
## [85] ""
## [86] "## positive constraints on loading diagonals"
## [87] "for (j in 1:n_lv) {"
## [88] "lv_coefs[j, j] ~ dnorm(0, 1)T(0, 1);"
## [89] "}"
## [90] ""
## [91] "## lower diagonal free"
## [92] "for (j in 2:n_lv) {"
## [93] "for (j2 in 1:(j - 1)) {"
## [94] "lv_coefs[j, j2] ~ dnorm(0, 1)T(-1, 1);"
## [95] "}"
## [96] "}"
## [97] ""
## [98] "## other elements also free"
## [99] "for (j in (n_lv + 1):n_series) {"
## [100] "for (j2 in 1:n_lv) {"
## [101] "lv_coefs[j, j2] ~ dnorm(0, 1)T(-1, 1);"
## [102] "}"
## [103] "}"
## [104] ""
## [105] "## trend evolution depends on latent factors"
## [106] "for (i in 1:n) {"
## [107] "for (s in 1:n_series) {"
## [108] "trend[i, s] <- inprod(lv_coefs[s,], LV[i,])"
## [109] "}"
## [110] "}"
## [111] ""
## [112] "## likelihood functions"
## [113] "for (i in 1:n) {"
## [114] "for (s in 1:n_series) {"
## [115] "y[i, s] ~ dnegbin(rate[i, s], r[s])"
## [116] "rate[i, s] <- ifelse((r[s] / (r[s] + mu[i, s])) < min_eps, min_eps,"
## [117] "(r[s] / (r[s] + mu[i, s])))"
## [118] "}"
## [119] "}"
## [120] ""
## [121] "## complexity penalising prior for the overdispersion parameter;"
## [122] "## where the likelihood reduces to a 'base' model (Poisson) unless"
## [123] "## the data support overdispersion"
## [124] "for (s in 1:n_series) {"
## [125] "r[s] <- pow(r_raw[s], 2)"
## [126] "r_raw[s] ~ dexp(0.05)"
## [127] "}"
## [128] ""
## [129] "## posterior predictions"
## [130] "for (i in 1:n) {"
## [131] "for (s in 1:n_series) {"
## [132] "ypred[i, s] ~ dnegbin(rate[i, s], r[s])"
## [133] "}"
## [134] "}"
## [135] ""
## [136] "## GAM-specific priors"
## [137] "## parametric effect priors (regularised for identifiability)"
## [138] "for (i in 1:1) { b[i] ~ dnorm(p_coefs[i], p_taus[i]) }"
## [139] "## prior for s(season)..."
## [140] "K1 <- S1[1:10,1:10] * lambda[1]"
## [141] "b[2:11] ~ dmnorm(zero[2:11],K1)"
## [142] "## prior for s(season,series)..."
## [143] "for (i in c(12:15,17:20,22:25,27:30)) { b[i] ~ dnorm(0, lambda[2]) }"
## [144] "for (i in c(16,21,26,31)) { b[i] ~ dnorm(0, lambda[3]) }"
## [145] "## smoothing parameter priors..."
## [146] "for (i in 1:3) {"
## [147] "lambda[i] ~ dexp(1/sp[i])"
## [148] "rho[i] <- log(lambda[i])"
## [149] "}"
## [150] "}"Inspection of the dynamic factors and their relative contributions indicates that the first factor is by far the most important
plot_mvgam_factors(trends_mod3)
Model 3 (with the dynamic trend) should provide far superior forecasts than relying only on the estimated smooths. Inspect the model summary (note again that p-value approximations are a work in progress here and so may not be totally reliable).
summary(trends_mod3)## GAM formula:## y ~ s(season, k = 12, m = 2, bs = "cc") + s(season, series, k = 5,
## bs = "fs", m = 1)## ## Family:## Negative Binomial## ## Link function:## log## ## Trend model:## AR1## ## N latent factors:## 4## ## N series:## 4## ## N observations:## 404## ## Status:## Fitted using runjags::run.jags()## ## Dispersion parameter estimates:## 2.5% 50% 97.5% Rhat n.eff
## r[1] 23.37430 198.0770 5479.860 1.09 3882
## r[2] 35.03135 489.3390 7020.542 1.01 4700
## r[3] 26.27291 275.1163 6163.969 1.01 3578
## r[4] 49.35871 451.5269 6511.605 1.01 4467## ## GAM smooth term estimated degrees of freedom:## edf df
## s(season) 8.029 10
## s(season,series) 10.597 5## ## GAM coefficient (beta) estimates:## 2.5% 50% 97.5% Rhat n.eff
## (Intercept) 1.56958341 2.43295864 3.493886669 1.84 25
## s(season).1 -0.60892972 -0.36191529 -0.138418213 1.02 60
## s(season).2 -0.93213092 -0.63674826 -0.351352392 1.04 71
## s(season).3 -0.84551620 -0.54160746 -0.235860974 1.02 70
## s(season).4 -0.66514912 -0.30655173 0.047298177 1.10 58
## s(season).5 -0.58630315 -0.24439017 0.090338636 1.10 50
## s(season).6 -0.29171699 0.03588541 0.345866090 1.06 48
## s(season).7 0.31297520 0.61636299 0.928603245 1.06 65
## s(season).8 0.47033855 0.76581669 1.058285205 1.05 63
## s(season).9 0.36800893 0.57986895 0.823639270 1.10 68
## s(season).10 -0.02612503 0.21235879 0.475981576 1.07 68
## s(season,series).1 -0.15518850 0.07193930 0.294568085 1.05 126
## s(season,series).2 -0.05431841 0.21222259 0.459805398 1.03 103
## s(season,series).3 -0.36429687 -0.12399712 0.138074665 1.08 56
## s(season,series).4 0.01915855 0.12744856 0.251575601 1.06 76
## s(season,series).5 -0.24382744 0.91935724 1.745845765 1.73 26
## s(season,series).6 -0.49061273 -0.21360552 0.055044796 1.03 261
## s(season,series).7 -0.42924137 -0.09457436 0.226953344 1.01 342
## s(season,series).8 -0.30018861 -0.03656410 0.240501055 1.07 100
## s(season,series).9 -0.16076629 -0.02930603 0.112174882 1.05 136
## s(season,series).10 -1.95760052 -0.72023112 0.003941407 1.41 81
## s(season,series).11 -0.10070694 0.13805008 0.381819701 1.05 159
## s(season,series).12 -0.43388134 -0.14186007 0.118163322 1.02 135
## s(season,series).13 -0.08802870 0.17352715 0.436913933 1.07 59
## s(season,series).14 -0.03246747 0.08212538 0.211140502 1.05 73
## s(season,series).15 -1.19391098 -0.04580932 0.981046821 1.79 28
## s(season,series).16 -0.35786580 -0.13074575 0.096699444 1.06 126
## s(season,series).17 -0.34619259 -0.06620841 0.183034820 1.03 111
## s(season,series).18 -0.21723226 0.02081461 0.292102547 1.08 58
## s(season,series).19 -0.13097801 -0.02386540 0.101013806 1.06 71
## s(season,series).20 -0.08634211 1.15708794 1.889027063 1.57 37## ## GAM smoothing parameter (rho) estimates:## 2.5% 50% 97.5% Rhat n.eff
## s(season) 4.307412 5.437207 6.338509 1.00 734
## s(season,series) 1.912014 2.712851 3.352031 1.01 2248
## s(season,series)2 -3.559197 -2.163150 -1.179041 1.00 8151## ## Latent trend parameter estimates:## 2.5% 50% 97.5% Rhat n.eff
## ar1[1] -0.2075121 0.17892866 1.0192503 4.10 2606
## ar1[2] -0.2319541 0.95769158 1.0230543 4.57 490
## ar1[3] -0.3319458 0.14465634 0.5880174 1.00 1562
## ar1[4] -0.5378748 0.04061721 0.6027934 1.00 1304## Look at Dunn-Smyth residuals for some series from this preferred model to ensure that our dynamic factor process has captured most of the temporal dependencies in the observations
plot_mvgam_resids(trends_mod3, series = 1)
plot_mvgam_resids(trends_mod3, series = 2)
plot_mvgam_resids(trends_mod3, series = 3)
plot_mvgam_resids(trends_mod3, series = 4)
Perform posterior predictive checks to see if the model is able to simulate data that looks realistic and unbiased by examining simulated kernel densities for posterior predictions (yhat) compared to the density of the observations (y). This will be particularly useful for examining whether the Negative Binomial observation model is able to produce realistic looking simulations for each individual series.
ppc(trends_mod3, series = 1, type = "hist")
ppc(trends_mod3, series = 2, type = "hist")
ppc(trends_mod3, series = 3, type = "hist")
ppc(trends_mod3, series = 4, type = "hist")
Look at traceplots for the smoothing parameters (rho)
plot_mvgam_trace(object = trends_mod3, param = "rho")
Plot posterior predictive distributions for the training and testing periods for each series
plot_mvgam_fc(object = trends_mod3, series = 1,
data_test = trends_data$data_test)
plot_mvgam_fc(object = trends_mod3, series = 2,
data_test = trends_data$data_test)
plot_mvgam_fc(object = trends_mod3, series = 3,
data_test = trends_data$data_test)
plot_mvgam_fc(object = trends_mod3, series = 4,
data_test = trends_data$data_test)
Plot posterior distributions for the latent trend estimates, again for the training and testing periods
plot_mvgam_trend(object = trends_mod3, series = 1,
data_test = trends_data$data_test)
plot_mvgam_trend(object = trends_mod3, series = 2,
data_test = trends_data$data_test)
plot_mvgam_trend(object = trends_mod3, series = 3,
data_test = trends_data$data_test)
plot_mvgam_trend(object = trends_mod3, series = 4,
data_test = trends_data$data_test)
Given that we fit a model with hierarchical seasonality, the seasonal smooths are able to deviate from one another (though they share the same wiggliness and all deviate from a common 'global' seasonal function)
plot_mvgam_smooth(object = trends_mod3, series = 1,
smooth = "season")
plot_mvgam_smooth(object = trends_mod3, series = 2,
smooth = "season")
plot_mvgam_smooth(object = trends_mod3, series = 3,
smooth = "season")
plot_mvgam_smooth(object = trends_mod3, series = 4,
smooth = "season")
Plot posterior mean estimates of latent trend correlations. These correlations are more useful than looking at latent factor loadings, for example to inspect ordinations. This is because the orders of the loadings (although constrained for identifiability purposes) can vary from chain to chain
correlations <- lv_correlations(object = trends_mod3)library(ggplot2)## Warning: package 'ggplot2' was built under R version 4.0.5mean_correlations <- correlations$mean_correlations
mean_correlations[upper.tri(mean_correlations)] <- NA
mean_correlations <- data.frame(mean_correlations)
ggplot(mean_correlations %>% tibble::rownames_to_column("series1") %>%
tidyr::pivot_longer(-c(series1), names_to = "series2",
values_to = "Correlation"), aes(x = series1,
y = series2)) + geom_tile(aes(fill = Correlation)) +
scale_fill_gradient2(low = "darkred",
mid = "white", high = "darkblue",
midpoint = 0, breaks = seq(-1, 1,
length.out = 5), limits = c(-1,
1), name = "Trend\ncorrelation") +
labs(x = "", y = "") + theme_dark() +
theme(axis.text.x = element_text(angle = 45,
hjust = 1))
There is certainly some evidence of positive trend correlations for a few of these search terms, which is not surprising given how similar some of them are and how closely linked they should be to interest about tick paralysis in Queensland. Plot some STL decompositions of these series to see if these trends are noticeable in the data
plot(stl(ts(as.vector(series$`tick paralysis`),
frequency = 12), "periodic"))
plot(stl(ts(as.vector(series$`paralysis tick dog`),
frequency = 12), "periodic"))
plot(stl(ts(as.vector(series$`dog tick`),
frequency = 12), "periodic"))
plot(stl(ts(as.vector(series$`tick bite`),
frequency = 12), "periodic"))
Forecast period posterior predictive checks suggest that the model still has room for improvement:
ppc(trends_mod3, series = 1, type = "hist",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 1, type = "mean",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 2, type = "hist",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 2, type = "mean",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 3, type = "hist",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 3, type = "mean",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 4, type = "hist",
data_test = trends_data$data_test)
ppc(trends_mod3, series = 4, type = "mean",
data_test = trends_data$data_test)
Other next steps could involve devising a more goal-specific set of posterior predictive checks (see this paper by Gelman et al and relevant works by Betancourt for examples) and compare out of sample Discrete Rank Probability Scores for this model and other versions for the latent trends (i.e. AR2, AR3, Random Walk)